An Application of Multivarble Techniques to an Electrical Power Plant

A."I

APPLICATION OF HULTIVARHBLE TECHNIQUES TO AN ELECTRICAL POWER PLANT

H. Dang Van Mien~ P. Godin~ C. Miossec~

The digital multivariable theory is used for a computeraided boiler control experiment in a 125 MW power plant. This experiment is led at the Pont-sur-Sambre power station in order to improve the control in different conditions of operation and disturbances.

INTRODUCTION This application covers the control of steam pressure and superheat and reheat temperatures by action on the fuel flow, the spray water flows, and the position of inside shutters for flue-gas distribution - The two load parameters are the throttle valve position and the steam flow - So, the model used for synthesis has four inputs and four outputs. During load changes, interactions observed in a boiler are multiple and very important, so that a multivariable control could be advantageous - We describe in this paper, the different stages of design and implementation, as well as the difficulties met during the experiment on the plant. The paper gives the results of the experiment for different load changes, and a comparison of control performance with classical analog control on the one hand, and digital single input single output control by z-transform algorithm on the other hand. Description of the Plant-Model of the Controlled System Diagrams of the 125 MW thermal unit will be found in Figure 1. Multivariable control is used for the main variables of the boiler, that Fuel flow (gas) (Cb) and the associated air flow (Qa) Control Total spray-water flow (Qi) variables Flue-gas shutters position (V T) Controlled variables

1S

Steam p~essure (Pv) Superheat temperature (Ts) Reheat temperature (Trs)

An identification of the system in March 1972 made it possible to obtain a model for preliminary study. This model, the step unit responses of which are given in Figure 2, is used in the form of a 120 points impulse response. The «Departement Automatique des Moyens de Production - E.D.F. CHATOU

215

throttle valve pos1t1on is a measurable variable but will not be controlled it is linked to the steam flow and to the pressure by a linear relation : Qv : steam flow

Os : valve position

Synthesis Method a) ~~~i£_EEi~£iE!~ The model described previously (in the form of an impulse response) is reduced by the B. L. HO Algorithm, which has been improved by a least squares method(2 J S)' This makes it possible to retain two final models of order 20 and order 9. The order 9 model enables the sensitivity study of the control system. The control synthesis method depends on optimal control with a quadratic criterion, according to KALMAN(A). Let a system be defined by the following state representation : 0) A-x.. +B..u. .!:1" C!.x. +.J)..u.. A ['7\. ' '"I'L] ,13(M. . m), CCl" . -n.J J l>Cf> _rrn.J The presence of drift on the system can be taken into consideration by adding a Wiener type noise to the inputs. Outputs will also be added disturbances. A complete description will therefore be written as follows :

x..,.

x,::

A x. +B (~+.".)

t\r=

'W"''''

~:. c.:x.+l>(M.+\Y)+w-t

j w 1 I w-~zerl)-mean white noise

(~)

When Wiener-type disturbances are present on the inputs, in order to obtain a finite criterion, input derivatives are introduced. The optimization criterion is then as follows: 1 = E ~ ~Jc;TQ e + .i.J.,' F'....u...) 0)

cLt}

where, e is the difference be\ween outputs and set-points. The solution of the problem of regulation or of the case of inputs intervening as disturbances, requires the definition of the probable behaviour of the set points or of the imcontrollable inputs. The following representation is used :

~c.

::0

~ ~c. - ~ l

with

'PE

~:. II,A.)-~

'VJ3: zero-mean white noise ;jE=[~c..1

as the dimension of the vector

given by

(5 )

Estimator

c) Simulation A si~~l;ti~~-study has given the weight coefficients for the optimization cr1terion. The use of Q and R matrices on the one hand and M and N matrices on the other hand (co-variance matrices of ' Wiener-type noise on inputs and disturbances on outputs) in the following form :

1

P=-p [ 1'. 'P O ="1'Po o 1't L

_• .

proved to be sufficient. Po is a value which will be modified to change the global control quality. Pi are weight coefficients concerning an input (or output) with respect to other inputs (or outputs). Pi are determined from input constraints or from output measurement precision. A study of the control sensitivity has been undertaken by simulation on a CDC 6600 computer by making the model parameters vary while blocking the controller.

216

Description of the processing system a) r~~_£~~E~E~E_~~~_iE~_~E~~~~E~_E~EiE~~E~l~

The T 2000 computer manufactured by the "Telemecanique S.A." was used for the experiment carried out at the Pont-sur-Sambre power station. The basic cycle of the central unit is 1.5 s. The word length is 20 bits. A memory protection system can be used for every word of the core memory. The processing system is composed of a 16 K words core memory, a 32 K words disk-memory, an ASR 33 teleprinter, a paper-tape reader and punch. b) ~~~l~g_~~~_~igiE~l_i~E~E:~~EE~E_~Y~E~~

The data acquisition system can operate at a rate of ISO channels per second. It comprises : - a Preston analog-digital converter - a 6 bit digital-analog converter - a multiplexing device providing filtering and good isolation of the inputs - a decoding unit - 60 analog inputs and 8 analog outputs - 38 digital inputs, including 12 inputs for the selection of the operating conditions : manual control-analog automatic control - digital automatic control c) ~~~~EY_~~~i£~~

To control the normal operation of the digital system of i~ase of failure, to give the control back to the existing analog regulation, the following equipment is available : - a watchdog system activated by

electric power failure - destruction of one of basic programs real time - failure of the time base system - failure of interruptiop.s tests - parity failure

- a supervision of analog outputs and inputs - a back-up system: the analog regulators fit the computer outputs before switching to analog control position. The switching the control the digital position 1S made when there is concordance between analog and digital signals - a device to give manual control always highest priority. Programming - real-time operation The theorical results given by the relation (5) are used In discrete form according to the following procedure : 1 ) Le t

X-= [-v..+..r

X be defined as :

~E by discretisation of/'0....relations (2, 4, 5) .,...._

First calculation

Xt.=Xl,+Kee~ rj>X~ ~T.A.ll.

X-;' : : ""+1

~-

~.ft.-M=

./'-.,..

eXe ~+.A

I

These relations are calculated before measurement of the outputs (Ymes)' After that, final calculation is carried out as follows

217

Second calculation

e

A..

~+~

=- j'mes -

:J -i+A

t1..u.hA = tJ.A...L{.,+A +

K

e,.,.""

Control variables are used in incremental form. For the 20 order model described previously, the first calculation lasts 2.2 seconds and the second calculation 0.2 second. The shift between measurements and controls is relatively slight (0,2 second). The sampling period used for control is 8 seconds. As an indication, the order 9 model requires 1.15 seconds for the first calculation under similar programmation conditions. For use in the site, gains (Ke, Kc, K ) and model (0, T, Ce ) are calculated on a CDC 6600 computer and punched on paper-tape which is used on the T 2000 computer. Many sets of coefficients (for different weights of Q, R, M, N) are used in the site. Results The first control tests use the ordre 20 model described above. They are not satisfactory and signs of pumping appear. Verification of the model, by comparing the latter with the real system, has shown that there were considerable differences for variations in the steam flow. Results of the identification procedure were not satisfactory and this involved some instability for the closed loop. A new identification compaign was necessary. The analysis was made by Messrs IRVING and BORGET ; the order 14 model obtained showed considerable improvement during verification operations carried out on the site. Two kinds of tests were adopted to test the control : a) - Qp.§~a!~0~ ~i~h_s.!!l~J 1. 102d_ v~~i~ti.oQ~ (fig. 4 and 4 bis) according to operating conditions with the network, variations are about 10 % of the nominal load. This is a continuous experiment over a period of one or more days. u) - ~t:..e'p "y~ri.a~~n_ 0"£ .!h~ J.o~
218

CONCLUSION The first results obtained are encouraging. Although they are not quite the same as those expected in the simulation study, it can be hoped that this method allows : overall synthesis, taking interaction systematically into account. - better dimensioning of feed forward control, enabling considerable improvements during important load changes or disturbances. The difficulties still lie on identification results. However, relatively low order models (about 10) seem to be sufficient for the control algorithm. Furthermore, programming does not present any practical problem of core memory capacity and occupation in time of the central unit. REFERENCES ]. KALMAN, R.E., BUCY, R.S. - 1961. "New results in linear filtering and prediction theory", Journal of Basic Engineering. 2. HO, B.L. and KALMAN, R.E. - 1966. "Effective construction of linear state variable models from input output fonctions", Regelungstechnik, 14. 3. IRVING, E., "Identification des systemes", Note interne de l'Electricite de France - Etudes et Recherches. 4. VAUGHAN, D.R. - 1970. "A non recursive algebraic solution for the discrete Riccati equation", IEEE Trans. on Automatic Control. 5. DANG VAN MIEN, H. - 1972. "Utilisation pratique de l'algorithme de B.L. HO". Rapport AMP 121, Electricite de France - Etudes et Recherches.

219

General spray water

2 nd spray water

1st spray water

second su perheater

first !>uperhllater

reheaters

Superheater temperature

final sup.rh.at.r

Sha m pressure

rehaater

Throttle valva

temperatu~

T

drum Shutter s for flue gas d istri bution

feed heater

irI

Sham flow

gas flow Feed pump fons

extraction pump

Fig .1_DESCR1PTlON OF POWER PLANT

220

Pv

Qv Steam flow

10 b per volt

40T/h per volt .90

0

.110

Throttle valve positi on

.eo \

.40

.... "F'"

.50 0

800

.4 0

Gas flow

.20

10

o

1600

2400

I

~ 1000

J\ I 2000

3000

V

.8

!

I

.6

I )1 4

1000

2000

3000

4000

I

.0

0

1000

2000

3000

,

.40

I

.30

I

.20

i

to

I 4COO

2000

3000

-30

4COO

~

.50

I

..

j

Vi 1000

.6 0

I

.2

20

30 0

4COO

I

V ~ -p-

~

.'"

JO

4

I

.1

f

0 .0

I

\

o

J

0

o \

-l 4

lIS

~

30

Qc

-I.

: i

a \

1

.50

1

6

20

I~

.to

I

4

.11)

Trs Reheater temperature 30·C per volt

Ts S u parheater Tem perature 30 DC per volt

Steam Pressure

0

1000

2llOO

3000

4COO

1.01-fl- \.-"l--+-+-+--I----l---l

1\

I

1\

I .41f-t-+-+-+-+--+-

.2H-+-+-+--+-+-+--I 0

1000

2000

3000

4COO

.00~-1OOO -':-:---'----:c2COO:':-:--'--:3000':-:---''-,-4000-=-'

o J)t;IJ

/,

.00011 H

Qj

Spray-water flow

J\

-.30 - .40

\' '-

.10

- .!lO

~.

040

o

1000

.0040

"\

o

!! " ! !

- .0040

/

2000

-,

3000

- .00II

2000

3000

-

,

1\

0 .0

1)\

\

\

.20

\

~

60 0

1000

2000

3000

-"'"0

4COO

Initia l

-

4000-

.4 0

0

\~p '" '000

--

2000

3000

4COO

i

J2OH-I-+-+-+-+--+-+--j

!

IYlO

I

- Dt60

3000

I

-

I;

2COO

\

/\ 1 \ ~ .....

0.0

,

.-

\: /'"

4000- .

.....,

-.""" i

I; ' COO

o.0

.010

distributron o

'000

4OCXi.200

:

-0120

-20

o

0.0 1)20

Fl ue -gas

10

fl

20

"

O .O ~-+-+-+-+--+-+--j

.rN1

tOOO

2000

3000

4000

0

1000

2000

3000

4000 0

'COO

model

Reduced model (order 20)

Fig . 2 _ STEP UNIT RESPONSES OF POWER PLANT

221

2000

3000

4000

Analog or digital control

Manual control

Manual actuactor

Manual

Switch

Automatic{analog)

-f----..

Automa tic (d igital) Warning light -1---'"

Fig.3 _ AUTOMATIC OR MANUAL CONTROL ACTUATOR

222

Switches

Oobit d. vap.ur

Stoom flow

Prrssur. s.t-point

Prt SS ion lOb

iiL :."

'1"

, ."

.... "

:r:' ....

·[ 'hq·r' ... .. ! "

Conoign. dOb it gaz

I,S '" Gas flow s.t point

Oobit fumi.

ofv

Gao flow :J:: ::

.•....• I','.·

Oobit injoction

{Ov Spray-wot ... flow

'. ' .....

. , -I':

.'1.

r

", TT:

j ' . ".

H 1::"1:':: .•• \ . 1

:< .... :., ... . r

I

i ·-!

" '1': :' , j

c..+II~.,. H,-'4"-+:-, .'

·· ''-···4. '''fl :''i " ·8 'if ','8'.+ ·.'4 ' .':" !H'B

I

L .

.....

~.

..•... ::

':;Y",::lli::LJ:: .'. _ . ." ... .....•,~

+. :.. " .. "." . ..... , -·:r'""j'p.':+++-" " ..,':i-'~LjJ'':'' 'i''''++-h+, "-+14 ' ·! ·.,,·+',· 1

H4L:

....

' ' j'

. i ;"" · I. '::':': ::.'::: '1.," I·.:"



:.. ..y::

Fig . 4_ EXPE RIME NT RESU LTS

223

( Fir 5 t ca 5 e )

. , i

. ···· \

Ttrmpiratur. ,urchaufh lO'C

Con,igne tempfroture ,urchouff. lO'C

Consign, debit injection lOY

Spray wot., flow set point

Reheat.,. temperature ut point

Tempif'atu,. r.,urchCNffe

Reheat ... temperature

RCM Commonde Mnt.ll.s lOY

Flue ,a' distribution

Fig.4bis_EXPERIMENT RESULTS (Firstcase)

224

25 Mw

_E!4--+" -+--JH--+-I

Stf'Om flow

r "', ""

________-'----__~':i~:;-+"(~-~--j + ,l W;:~Iu-!--4 ~M- ' liot .,

l.~ IIIoW

,

~4~ )~~1)-l

Con,ion. prusion

,I::,

'i ~

..

lOb

Pru,ion

lOb

--+

------------------------~------" ~~~_L~~fL-L-~~--+-~-L" l

--+--+-+--+--+--+-II+-+-->---;-+-+--+--+--+-+-+--,--+-+-+--+--+--+-+t-+-->---;-+-+--+--+--+ -- -I--

-

....

--+--t -{ -

+--+

\--i-~;+' -H--b , + L'-!L-l-H-W+ ;"+-+-~jt=l= -

Fig . 5 _ EX PER IMENT

RESULTS (Second

225

caseJ

I:-±++-;",="

T.rnpforatur. surchaufh

...

1

.. -

30·C

b.

Sup.rh.at.r t.mp.raturt

bQ.i~~e

d. cl.u~~ H-' "'I
m.o .... ~t~

d.
Consign. temperature surchaufh

,

30·C

"

MW

..

Superheater temperature ut po int

Consigne debit inject 'on IOV

.....

I"\;'

I
Spray wot., flow set point

Consigne temperature risurchoufft 30·C

Reh.ater temp.rature ut point

-

Tempiratur. r.surchouffa 3d\:

Reheahr t.mperature

.

. ?,'

ReM Commande "ent.l1al

~

,.1 '

-.< IOV

.

Flue IQI distribut i on

+-

Fig. 5bis _ EXPERIMENT RESULTS

226

(Second

case)